The present invention relates to aircraft altitude instruments, and more particularly to an apparatus for providing a higher resolution indication of aircraft altitude than is presently available from conventional devices.
Aircraft altimeters conventionally operate under the principle of measuring the output of a pressure transducer on the aircraft, after adjustment for local ground barometric pressure variations, and converting the pressure measurement into an altitude represented on a meter or dial in feet. Because of certain non-linearities in the various parameters measured, and including non-linearities caused by temperature, the prior art has utilized various forms of introducing correction for non-linearity.
The conventional aircraft altitude encoder, known as a "mode C" encoder, typically generates an 11-bit binary transmission code which is representative of altitude. For each 100-foot change in measured altitude, the encoder output signals change, utilizing a gray code convention, so that each code change is representative of a transition point in altitude measurement. The output from the conventional mode C encoder is typically interfaced with a radio transmitter or transponder that transmits the altitude signal on request to a ground air traffic control center, along with the aircraft identification. This information, after adjustment for local ground barometric pressure variations, may then be mapped on a radar screen in the control center, and is used to keep track of air traffic in a controlled zone. Of course, the altitude reading may also be presented on a suitable meter in the aircraft cockpit.